This invention relates to surface acoustic wave devices and to a substrate of lead potassium niobate, Pb.sub.2 KNb.sub.5 O.sub.15, (PKN) for use therewith.
ST-cut quartz is often utilized as a piezoelectric substrate material for a wide variety of surface acoustic wave devices (SAW) such as filters, delay time encoders, decoders, correlators as well as other signal processing devices. Unfortunately, ST-cut quartz possesses a low piezoelectric coupling constant and, therefore, is not suitable for use in SAW devices designed to have low insertion losses and broad bandwidths. As a consequence, a considerable research effort has evolved in an attempt to find other materials for use as SAW substrates that are temperature compensated and possess a requisite high piezoelectric coupling constant. In attempting to find desirable materials, it was determined that, in order to be temperature compensated, the material often possesses either a positive temperature coefficient of an elastic constant or a negative coefficient of thermal expansion. That such a concept is valid has been demonstrated by the results of recent calculations of the SAW properties of berlinite (which has a positive temperature coefficient of an elastic constant) and .beta.-eucryptite (which has a negative coefficient of thermal expansion). Those calculations showed that both materials are indeed temperature compensated and have larger piezoelectric coupling constants than ST-cut quartz.
However, berlinite and .beta.-eucryptite still fail to possess as low a diffraction and as high piezoelectric constant as is desired for certain SAW applications. In further attempts at finding materials which might prove useful and desirable for SAW applications, it was discovered that lead potassium niobate (PKN), which occurs in the tungsten bronze structure and belongs to the orthorhombic crystal class mm.sub.2 (C.sub.2v) is attractive for SAW applications, when provided crystellographically oriented along the Y-axis in the manner, described herein. The most significant feature of this material is that its piezoelectric coupling constant is up to 16 times as large as that of ST-cut quartz, a material utilized heretofore for SAW devices. In addition, the diffraction spreading of PKN is less than that of an isotropic material, an attractive feature not shared by either quartz or berlinite. Calculations undertaken during the research effort have shown that a particular substrate of PKN having the Y-axis as the direction of propagation provides high piezoelectric coupling with a coupling constant 16 times as large as that of ST-cut quartz. The particular crystallographic orientation of this invention for a Y-axix boule orientation is defined by the Euler angle: Lambda=90.0.degree.; Mu=66.6.degree.; and Theta=0.0.degree..
Currently, lithium niobate (LiNbO.sub.3) is used as a substrate material in surface acoustic wave devices requiring greater bandwidth (for a given amount of insertion loss) than that obtainable with ST cut quartz. But because LiNbO.sub.3 has a large sensitivity to temperature, bulky and costly ovens are required for temperature control. This new orientation of PKN will extend to range of device bandwidths possible without having to resort to lithium niobate with its associated ovens and other temperature controlling schemes.
The most important feature of the crystallographic orientation of this invention is that its piezoelectric coupling coefficient is 16 times as large as that of ST-Cut quartz. This makes it possible to build low insertion-loss SAW devices with low temperature sensitivity and larger bandwidths than those obtainable in the devices currently being built on ST-cut quartz.